The present invention relates to a nitride semiconductor device such as a blue laser used for high-density optical recording.
FIG. 14 illustrates a nitride semiconductor laser device disclosed in Japanese Laid-Open Patent Publication No. 8-153931. This semiconductor laser device includes a sapphire substrate 1 using the (0001) surface as its main surface and a layered structure formed thereon. The layered structure is formed by sequentially growing a first cladding layer 2, an active layer 4, and a second cladding layer 6, which are all made of nitride semiconductor layers using the (0001) surface as their main surfaces. The layered structure is then cut along a surface equivalent to the (1-100) surface of the sapphire substrate 1, to obtain an optical resonator surface. Note that as used herein “−1” refers to the inversion of “1” and “−2” refers to the inversion of “2”.
The conventional nitride semiconductor laser device has a problem as follows. Since the sapphire substrate 1, which is made of hexagonal crystal, uses the (0001) surface as its main surface, there exist only a small number of atomic steps on the surface of the substrate. This tends to cause composition separation in the active layer 4. Occurrence of composition separation in the active layer 4 results in increase in the threshold current of the laser and thus increase in power consumption. It is therefore necessary to minimize occurrence of composition separation in the active layer 4.
The above problem arises not only in a nitride semiconductor laser having an active layer, but also in a nitride semiconductor device having a function region other than the active layer, such as a transistor.